Jim has tried on several occasions to break out of his usual slapstick comedy that at one point in time made him the highest paid actor ever. I suppose when you have that much money and success you aspire to do something different and I can't blame the guy for that. Sandler tries to do it from time to time as well. For the most part Jim Carrey has been extremely unsuccessful doing anything other than his usual comedies - this was an exception and in turn is an exceptional film.
What I am about to say may seem like spoilers but it is the entire premise of the film, it is slowly revealed in the start and it actually directly referenced in the trailer - so no, it is not spoilers :)
Carrey plays Joel Barish who is a shy, introverted dude with not many friends who encounters Clementine Kruczynski (Kate Winslet) on a train one day and they immediately connect with one another in an uncanny way. This is also combined with the fact that both of them happen to be going to the same train stop and neither one are really certain why they need to go there.
The reason they hit it off so quickly is because they actually do know eachother and have been in a long-term relationship yet they both had gone to a clinic that specializes in the removal of painful memories from a person's brain. Basically, they had erased one another.
Most of the movie takes place inside of Joel's brain and this is the reason why the movie takes on paranormal or even thriller-esque qualities. It is not meant to be scary, but can't help but be at certain times. Someone with a greater command of film-school words could probably put better words to this but the choice of scenery and camera angles are very unusual and dreamlike, as you would suspect since most of the movie takes place in a "dream" of sorts where the laws of physics do not apply.
It isn't terribly surprising that Winslet gives a tremendous dramatic performance and she was nominated for many awards for this, including the Oscar for best actress (she didn't win, but it an honor just being nominated... right?) but the real surprise is the fact that Carrey does a tremendous job being serious as well. He has never been able to recapture this emotion and he has tried - i suppose he came close in "Man on the Moon."
This story is touching but it is also quite realistic as it shows a couple doing normal couple things and having their eccentricities, which were part of their initial attraction, become something of an irritation later, which is pretty common in relationships - at least it has been for me. Anyway, the artistic, creative, and innovated way in which this story is told combined with the fact that Carrey not being well, what he always is... makes this movie incredible and I consider it s a must-see.